Position regulators are used in installations for the control of valves in order to position the travel of stroke or the opening h of the control valve in accordance with a desired parameter. This parameter consists, as a rule, of a given desired flow quantity Q. Interfering and frictional forces in the flow should to be eliminated to a great extent. A given opening h of the valve is therefore associated with a given flow quantity Q. This ratio Q(h) is generally called a working characteristic.
In order to obtain the valve opening h corresponding to a desired flow quantity Q, a controller output position w is fed to the position regulator in the form of an electrical current signal and is compared with the topical opening or lifting position h of the valve. Depending on the result of this comparison, a pneumatic or mechanical positioning force is then fed to the pneumatic positioning drive until position and regulated magnitude (desired-/actual value) coincide.
The conventional position regulators are usually made according to electro-mechanical principles. The travel of stroke h or the opening of the valve is detected mechanically, e.g. through a lever coupling, and is fed back into the position regulator.
Newer designs replace the electro-mechanical system with electronic components, with microprocessors also used. The valve opening h is, in that case, converted into an electrical magnitude, e.g. by a potentiometer, and is compared in a suitable manner, e.g. through the processor software, with the guiding magnitude. These position regulators operate as microprocessor-controlled system as follows:
The controller output w is provided in the form of a current or voltage signal, e.g. by a process computer or also process regulator which calculates the position magnitude W, i.e. signals to trigger the valve via an algorithm formed in the microprocessor-controlled circuit of the computer (calculating process). The processing of the signals as a command and regulating variables is effected via a microprocessor-controlled circuit of the position regulator. Finally the obtained travel of stroke h is ascertained and is fed back via a potentiometer or a contact-less distance measuring system.
It is known that every valve has a certain characteristic valve line K.sub.v (h), which changes however relative to the working characteristic Q(h) depending on the resistances and operating states of the installation into which the valve is built in. For the precision, stability and regulating quality of the regulating circuit of the installation in which the valve is used as a positioning element, it would however be desirable to provide the same travel of stroke changes for the same flow changes. In other words: The working characteristic Q(h) of the system should show the flow Q as a linear function of travel of stroke h, i.e. for all operating states.
It has already been tried to assimilate, at least as much as possible, a linear working characteristic Q(h) by means of chokes, i.e. through mechanical configuration of the valves. However, this is very expensive. Above all, the form of the valve, once it has been designed, is only able to produce a linear working characteristic for one particular operating state. With other operating states and also in other installations, the design of this valve must again be adapted. Also, only an approximately linear attitude is obtained in this manner.